Suspension-adapted Chinese hamster ovary-derived cells expressing green fluorescent protein as a screening tool for biomaterials.

Synthetic biomaterials play an important role in regenerative medicine. To be effective they must support cell attachment and proliferation in addition to being non-toxic and non-immunogenic. We used a suspension-adapted Chinese hamster ovary-derived cell line expressing green fluorescent protein (GFP) to assess cell attachment and growth on synthetic biomaterials by direct measurement of GFP-specific fluorescence. To simplify operations, all cell cultivation steps were performed in orbitally-shaken, disposable containers. Comparative studies between this GFP assay and previously established cell quantification assays demonstrated that this novel approach is suitable for rapid screening of a large number of samples. Furthermore the utility of our assay system was confirmed by evaluation of cell growth on three polyvinylidene fluoride polymer scaffolds that differed in pore diameter and drawing conditions. The data presented here prove the general utility of GFP-expressing cell lines and orbital shaking technology for the screening of biomaterials for tissue engineering applications.

Effects of methotrexate on transfected DNA stability in mammalian cells.

The chromosomal locations, amounts, and level of expression of transfected, amplified c-myc and dihydrofolate reductase sequences were measured in cells cultured in the presence and absence of methotrexate. These studies show that the location and amount of transfected sequences, as well as the level of expression, were more variable when the cells were cultured in methotrexate.

A variant of t-PA (T103N, KHRR 296-299 AAAA) that, by bolus, has increased potency and decreased systemic activation of plasminogen.

In the accompanying paper, we reported that the properties of decreased plasma clearance rate, increased fibrin specificity, and resistance to inactivation by PAI-1 could be effectively combined in the t-PA variant T103N, KHRR 296-299 AAAA. In the current study we evaluated the in vivo efficacy of this variant as well as variants containing the individual mutations T103N and KHRR 296-299 AAAA. Plasma clearance and in vivo lysis of whole blood and platelet-rich clots were determined in a rabbit arterio-venous shunt model. The T103N containing variants were administered as an intravenous (i.v.) bolus. KHRR 296-299 AAAA and t-PA were infused i.v. over 90 min. The clearance rate of the KHRR 296-299 AAAA variant was similar to t-PA. However, the clearance of the T103N and T103N, KHRR 296-299 AAAA variants were 8 and 6-fold reduced, respectively. Potency of the variants relative to t-PA on whole blood clots ranged from 0.9 (T103N, KHRR 296-299 AAAA) to 1.7 (T103N). Relative potency on platelet-rich clots ranged from 2.4 (T103N) to 4.2 (T103N, KHRR 296-299 AAAA). Fibrinogen concentrations in rabbits 120 min after dosing with a 2.5 mg/kg bolus were: 24, 16, 82, and 77% of initial for t-PA; T103N; KHRR 296-299 AAAA; and T103N, KHRR 296-299 AAAA treatment groups, respectively. These results suggest that the T103N, KHRR 296-299 AAAA variant of t-PA, given as a bolus, could result in greater efficacy, particularly on refractory platelet-rich clots, without inducing the severe systemic lytic state produced by a bolus of a less fibrin specific variant.

A slow clearing, fibrin-specific, PAI-1 resistant variant of t-PA (T103N, KHRR 296-299 AAAA).

Site directed mutagenesis was used to construct a t-PA variant that contains an additional glycosylation site in the first kringle domain (T103N) combined with a tetra-alanine substitution in the protease domain (KHRR 296-299 AAAA). This combination variant has a plasma clearance rate that is 4.5-fold slower in rats and 5.4-fold slower in rabbits than t-PA. It is also less than one tenth as active as t-PA towards plasminogen in the presence of fibrinogen, and has approximately twice the normal activity in the presence of fibrin. It shows substantial resistance to the fast acting inhibitor, plasminogen activator inhibitor-1 (PAI-1), requiring a 10-fold greater molar excess of PAI-1 to reduce its activity by 50%, compared to t-PA. This is the result of a reduction of nearly 100-fold in the second order rate constant for PAI-1 inactivation. These results show that it is possible to combine mutations in different domains of t-PA to construct a variant which is simultaneously slower clearing, less reactive towards plasminogen in the absence of a fibrin clot, and resistant to inactivation by PAI-1.

Transfection of partially purified plasmid DNA for high level transient protein expression in HEK293-EBNA cells.

One of the major constraints to performing large-scale transfections of cultured mammalian cells for the transient expression of recombinant proteins is the production of large quantities of purified plasmid DNA. In this report partially purified plasmid DNA was prepared by a method that combines alkaline lysis of E. coli with standard precipitation techniques. The efficiency of calcium phosphate-DNA co-precipitate formation with crude DNA was similar to that observed for pure DNA, but precipitate formed with crude DNA also contained RNA. The transfection of adherent and suspension-adapted HEK293-EBNA cells with partially purified pEGFPN1 resulted in levels of transient GFP expression equivalent to those achieved with pure DNA. In addition, the co-transfection of 1-200 ml cultures of suspension-adapted HEK293-EBNA cells with two different plasmids encoding the heavy and light chain genes of anti-human RhD IgG1, respectively, yielded similar IgG titers with pure and partially purified plasmid DNA. Finally, it was observed that suspension-adapted cells were more tolerant to the presence of RNA in the plasmid preparations than were adherent cells. These findings are relevant to the field of DNA transfection, including applications ranging from high-throughput screening to large-scale transient protein expression.

Small-scale bioreactor system for process development and optimization.

An agitated 12-well microtiter plate system with a working volume of 2ml was investigated for cell culture process development. Agitation assures homogeneity in wells and enhances mass transfer between the gas and the liquid phase, thus improving maximum cell density and pH stability. The pH of the NaHCO(3)-buffered system can be adjusted by altering the carbon dioxide content of the gas phase. The non-toxic, visual pH indicator phenol red was used in combination with a spectrophotometric plate reader for rapid and precise pH measurements. For high throughputs, cell growth was assessed non-invasively using stable green fluorescent protein (GFP) expressing cells and a fluorescence plate reader. The setup is simple and inexpensive. The system can be automated and allows several hundred small-scale bioreactor experiments to be run in parallel.

Compressed collagen gel: a novel scaffold for human bladder cells.

Collagen is highly conserved across species and has been used extensively for tissue regeneration; however, its mechanical properties are limited. A recent advance using plastic compression of collagen gels to achieve much higher concentrations significantly increases its mechanical properties at the neo-tissue level. This controlled, cell-independent process allows the engineering of biomimetic scaffolds. We have evaluated plastic compressed collagen scaffolds seeded with human bladder smooth muscle cells inside and urothelial cells on the gel surface for potential urological applications. Bladder smooth muscle and urothelial cells were visualized using scanning electron microscopy, conventional histology and immunohistochemistry; cell viability and proliferation were also quantified for 14 days in vitro. Both cell types tested proliferated on the construct surface, forming dense cell layers after 2 weeks. However, smooth muscle cells seeded within the construct, assessed with the Alamar blue assay, showed lower proliferation. Cellular distribution within the construct was also evaluated, using confocal microscopy. After 14 days of in vitro culture, 30% of the smooth muscle cells were found on the construct surface compared to 0% at day 1. Our results provide some evidence that cell-seeded plastic compressed collagen has significant potential for bladder tissue regeneration, as these materials allow efficient cell seeding inside the construct as well as cell proliferation.

Genetic characterization of CHO production host DG44 and derivative recombinant cell lines.

The dihydrofolate reductase-deficient Chinese hamster ovary (CHO) cell line DG44 is the dominant mammalian host for recombinant protein manufacturing, in large part because of the availability of a well-characterized genetic selection and amplification system. However, this cell line has not been studied at the cytogenetic level. Here, the first detailed karyotype analysis of DG44 and several recombinant derivative cell lines is described. In contrast to the 22 chromosomes in diploid Chinese hamster cells, DG44 has 20 chromosomes, only seven of which are normal. In addition, four Z group chromosomes, seven derivative chromosomes, and 2 marker chromosomes were identified. For all but one of the 16 DG44-derived recombinant cell lines analyzed, a single integration site was detected by fluorescence in situ hybridization regardless of the gene delivery method (calcium phosphate-DNA coprecipitation or microinjection), the topology of the DNA (circular or linear), or the integrated plasmid copy number (between 1 and 51). Chromosomal aberrations, observed in more than half of the cell lines studied, were mostly unbalanced with examples of aneuploidy, deletions, and complex rearrangements. The results demonstrate that chromosomal aberrations are frequently associated with the establishment of recombinant CHO DG44 cell lines. Noteworthy, there was no direct correlation between the stability of the genome and the stability of recombinant protein expression.

Plasmid integration, amplification and cytogenetics in CHO cells: questions and comments.

This paper addresses seven questions with respect to the transfer, integration, amplification and genetic stability of recombinant DNA in mammalian genomes. Most of these questions were raised with those issues in mind which are frequently discussed in the context of the manufacture of biologicals of therapeutic value on the basis of recombinant cell lines. For the most part, a high degree of ignorance has to be acknowledged and only very limited fragments of information are available. The reason for this ignorance is, as will become clear from this article, the sheer size and complexity of the mammalian genome and the inadequacy of presently available tools to unravel this complexity.

Extraction of plasmid DNA using reactor scale alkaline lysis and selective precipitation for scalable transient transfection.

DNA extracted and purified for vaccination, gene therapy or transfection of cultured cells has to meet different criteria. We describe herein, a scalable process for the primary extraction of plasmid DNA suitable for transient expression of recombinant protein. We focus on the scale up of alkaline lysis for the extraction of plasmid DNA from Escherichia coli, and use a simple stirred tank reactor system to achieve this. By adding a series of three precipitations (including a selective precipitation step with ammonium acetate) we enrich very quickly the plasmid DNA content in the extract. The process has been thus far used to extract up to 100 mg of plasmid from 1.5 l of clarified lysate, corresponding to an E.coli bioreactor fermentation of 3 l.

Integration and stability of CHO amplicons containing plasmid sequences.

Fluorescence in situ hybridization (FISH, 15) and high density, non-selective long term perfusion culture were used to study aspects of genetic stability and productivity in recombinant CHO cells. We analysed the distribution and structure of recombinant amplicons in chromosomes of CHO cells used for the production of proteins. In the presence, but not in the absence, of methotrexate (MTX) we found a high proportion of cells (40-60%) with multiple and/or unusually structured and extended chromosome regions containing amplified sequences. Removal of MTX from culture media resulted in the rapid decline in the frequency of cells containing amplified sequences exhibiting multiple and heterogeneous integrations. In cloned lines cultivated in the absence of MTX, a well defined signal motif on a specific chromosome, interpreted as the "master integration" unit, became increasingly abundant over time until almost all cells contained that signal motif. We used fluorescence in situ hybridization (FISH) with chemically modified DNA probes complementary to the integrated sequences to verify the stability of master integrations in cells cultured for extended periods in medium lacking MTX. In a second approach, we studied long-term stability of recombinant sequences during non-selective perfusion culture of CHO populations with non-cloned, MTX resistant heterogeneous populations of cells producing CD4IgG chimeric molecules. Due to the mode of transfection and primary selection the resulting cell lines consisted of subpopulations of cells derived from various independent integration events. The amplification and MTX selection procedure used consecutively generated multiple, structurally different amplicon types in the cell population, thus increasing the degree of heterogeneity. High density perfusion culture of these cells in the absence of MTX was used to maximize growth rates and was thought to select against cells with reduced growth rates, due to the highly amplified state of their introduced DNA, with concomitantly high productivity. However we found no evidence for such a selection; cells showed no reduction in copy number or total loss of amplified sequences at the end of the culture. More significantly, specific productivity of these cell lines grown under non-selective conditions did not decrease over the 100 days observation period.

Inducible overproduction of the mouse c-myc protein in mammalian cells.

We have made Chinese hamster ovary (CHO) cell lines that contain up to 2000 copies of the coding region of the mouse c-myc gene fused to the promoter of the Drosophila gene (hsp70) encoding a Mr 70,000 heat shock protein. Incubation of these cells at 43 degrees C results in an estimated 100-fold induction of c-myc mRNA. Translation of this mRNA occurs when the cells are returned to 37 degrees C, and during the first 3 hr of recovery at 37 degrees C, the c-myc protein is one of the most abundantly synthesized proteins in the cells. The products of the induced c-myc gene are phosphoproteins of apparent Mr 64,000, 66,000, and 75,000. Induced cells die, suggesting that elevated levels of c-myc are cytotoxic. Amplification of genes placed under control of the Drosophila hsp70 promoter may provide a general method for inducibly over expressing proteins in mammalian cells.

Calcium-phosphate mediated DNA transfer into HEK-293 cells in suspension: control of physicochemical parameters allows transfection in stirred media. Transfection and protein expression in mammalian cells.

This is the first report of transient transfection of suspended cells with purified plasmid DNA in bioreactors or spinner flasks. DNA/calcium phosphate complexes were pumped or injected directly into stirred cultures of the immortalized human embryo kidney cell line 293 (HEK-293) which had been adapted to growth in suspension. We identified culture conditions suitable for this approach and modified the protocol for the generation of precipitate complexes, based on our earlier work. In order to stabilize the 'DNA-vehicle'-complex in the culture medium, we identified pH ranges and ion-concentrations which prevent dissolution or aggregation of the precipitate particles. Such conditions maintained suspended fine particles in spinners or bioreactors for up to 6 hr. During that period, cells and precipitate complexes interacted sufficiently to allow DNA transfer and subsequent expression of recombinant protein. In a simple 5 day batch process, with a starting density of 0.3 × 10(6) cells mL(-1), about 0.5 mg L(-1) of a recombinant tissue plasminogen activator variant was observed.

Use of fluorescence in situ hybridization to detect and monitor transfected and amplified sequences in recombinant CHO cells.

Non-radioactive fluorescence in situ hybridization was used to detect transfected and amplified sequences in recombinant CHO cells. The cell lines used in this study are based on DHFR-deficient CHO-DUKX cells. The recombinant CHO cells contain and express, as verified by Southern and Northern experiments, multiple copies of a constitutive DHFR expression vector, as well as an inducible Drosophila HSP 70 promoter-mouse c-myc construction. In order to localize and monitor the chromosomal location of transfected and amplified DHFR and c-myc sequences, biotinylated DNA probes were hybridized to metaphase preparations of several cell lines. The resulting hybrids were detected using fluorescein-linked avidin. The fluorescence signal was amplified using a biotinylated anti-avidin antibody. The number of c-myc and DHFR integration sites per metaphase, their distribution in cell populations growing at various methotrexate levels, the sizes of the amplified sequences, as well as the number of chromosomal rearrangements were measured. The results of this study will be presented and the usefulness of this method as a general tool for rapid characterization and monitoring of recombinant cell lines will be discussed.

Transfecting mammalian cells: optimization of critical parameters affecting calcium-phosphate precipitate formation.

DNA-calcium phosphate co-precipitates arise spontaneously in supersaturated solutions. Highly effective precipitates for transfection purposes, however, can be generated only in a very narrow range of physico-chemical conditions that control the initiation and growth of precipitate complexes. The concentrations of calcium and phosphate are the main factors influencing characteristics of the precipitate complex, but other parameters, such as temperature, DNA concentration and reaction time are important as well. An example for this is the finding that almost all of the soluble DNA in the reaction mix can be bound into an insoluble complex with calcium phosphate in <1 min. Extending the reaction time to 20 min results in aggregation and/or growth of particles and reduces the level of expression. With improved protocols we gained better reproducibility and higher efficiencies both for transient and for stable transfections. Up to 60% of cells stained positive for beta-gal and transient production of secreted proteins was improved 5- to 10-fold over results seen with transfections using standard procedures. Similar improvements in efficiency (number of recombinant cell colonies) were observed with stable transfections, using co-transfected marker plasmids for selection. Transient expression levels 2 days after DNA transfer and titers obtained from stable cell lines, emerging weeks later, showed strong correlation.

Calcium phosphate transfection optimization for serum-free suspension culture.

Aim of this study was to identify optimal conditions for suspension transfection in the absence of serum. Transfection parameters for suspension culture can be very different to ones in adherent cells. Most transfection protocols have been developed and optimizedfor adherent culture. Using green fluorescent protein (GFP) as reporter, FCS was eliminated from the transfection process by altering critical parameters and by substituting serum with albumin. Using standard phosphate and calcium concentrations for transfection in the absence of serum resulted in titers of only 1% of those observed in the presence of serum. A reduction of the calcium concentration from 250 mM to 100 mM, yielded a 25-fold increase in the expression of the recombinant protein compared to the serum-free standard conditions. Altering the phosphate concentration, 1.4 mM in the transfection buffer, did not improve the protein expression. Interestingly, reduction of DNA quantity by half to a concentration of 0.5 µg per milliliter of culture volume resulted in a two-fold increase of protein production. Addition of albumin to serum-free medium protected the cells against the toxicity of the calcium phosphate transfection particles (CaPi) yielding higher protein expression. All the experiments were executed in a shaken multi-well system, allowing high multiplicity parameter screening to speed up optimizations. The culture system is inexpensive, simple and efficient, minimizing costs for labor and consumables.

A region of tissue plasminogen activator that affects plasminogen activation differentially with various fibrin(ogen)-related stimulators.

The dissolution of blood clots by plasmin is normally initiated in vivo by the activation of plasminogen to plasmin through the activity of tissue plasminogen activator (t-PA). The rate of plasminogen activation can be stimulated several orders of magnitude by the presence of fibrin-related proteins. Here we describe the kinetic analysis of both recombinant human t-PA (wild-type) and a t-PA variant produced by site-directed mutagenesis in which the original sequence from amino acids 296 to 299, KHRR, has been altered to AAAA. This tetra-alanine variant form of t-PA, K296A/H297A/R298A/R299A t-PA, we refer to as "KHRR" t-PA here. The plasminogen activating kinetics of wild-type t-PA (Activase alteplase) showed a catalytic efficiency which changed over 100-fold dependent on the stimulator in the assay. The lowest rate was in the absence of a stimulator. The following stimulators showed increasing ability to accelerate the catalytic efficiency of the reaction: fibrinogen, fragments of fibrinogen obtained by digestion with plasmin, fibrin, and slightly degraded fibrin. This increase in efficiency was driven primarily by decreases in the Michaelis constant (KM) of the reaction, whereas the catalytic rate constant (kcat) of the reaction did not change significantly. The "KHRR" variant of t-PA displayed novel kinetics with all stimulators tested. In the absence of a stimulator or with the poorer stimulators (fibrinogen and fibrinogen fragments), the KM values of the reaction with Activase alteplase and "KHRR" t-PA were similar. The kcat however, was lower with "KHRR" t-PA than with wild-type t-PA.(ABSTRACT TRUNCATED AT 250 WORDS)

GFP-expressing mammalian cells for fast, sensitive, noninvasive cell growth assessment in a kinetic mode.

This study correlates the fluorescent signal from stable recombinant CHO cell lines expressing the green fluorescent protein (GFP) at high levels with biomass or cell number, extending the use of fluorescent proteins to applications and assays where cell growth rates are important. Using a standard fluorometer, growth of these cells can be quantified noninvasively in multiwell plates, and because signals are obtained without preparation, the same culture samples can be measured repeatedly. Even with a small relative change in biomass, the specific growth rate can be determined in a few hours. The dynamics of cell populations can now be studied with high sensitivity, low error rate, and minimum sample preparation.

Transient gene expression: recombinant protein production with suspension-adapted HEK293-EBNA cells.

Transient gene expression (TGE) in mammalian cells at the reactor scale is becoming increasingly important for the rapid production of recombinant proteins. We improved a process for transient calcium phosphate-based transfection of HEK293-EBNA cells in a 1-3 L bioreactor volume. Cells were adapted to suspension culture using a commercially available medium (BioWhittaker, Walkersville, MD). Process parameters were optimized using a plasmid reporter vector encoding the enhanced green fluorescent protein (EGFP/CLONTECH, Palo Alto, CA, USA). Using GFP as a marker-protein, we observed by microscopic examination transfection efficiencies between 70-100%. Three different recombinant proteins were synthesized within a timeframe of 7 days from time of transfection to harvest. The first, a human recombinant IgG(1)-type antibody, was secreted into the supernatant of the cell culture and achieved a final concentration of >20 mg/L. An E. coli-derived DNA-binding protein remained intracellular, as expected, but accumulated to such a concentration that the lysate of cells, taken up into the entire culture volume, gave a concentration of 18 mg/L. The third protein, a transmembrane receptor, was expressed at 3-6 x 10(6) molecules/cell.